The first three-dimensional images of the Moon's poles have taken using radar.

The observations were made with Nasa's Goldstone Deep Space Network radio telescopes and reveal deep craters in permanent shadow that could potentially contain water ice. The south pole appears particularly suitable.

The cold traps appear white at the Moon's south pole

The new data will be vital in selecting and hitting a target for the crash landing of the orbiting Lunar Prospector spacecraft on 31 July. This was also announced on Wednesday and hopes to blast water vapour high enough out of a crater for observatories to confirm its presence.

"In order to impact the spacecraft at the desired location, very accurate knowledge of the topography is needed," says Donald Campbell, professor of astronomy at Cornell University, one of the team who created the maps.

Hydrogen clue

In 1996, researchers working with radar data from the orbiting lunar spacecraft Clementine reported indications of ice at the south pole of the Moon.

Last year, the neutron spectrometer aboard the Lunar Prospector orbiter, launched in January 1998, detected significant deposits of hydrogen at the Moon's north and south poles.

Cold traps at the north pole are less common

This was interpreted as indicating the presence of water ice, since hydrogen in water molecules is thought to be the most likely source of the element at the poles.

Cold Traps

The new, detailed topographic maps of the poles have made it possible to identify the potential ice-containing regions, called "cold traps". These are areas where the sun never shines and the temperature hovers around 100 Kelvin (-173 degrees Celsius).

The sun's limb rises less than two degrees above the horizon at the south pole, meaning the floors of impact craters and other low areas can be in permanent shadow.

In contrast, the radar beam from Goldstone reaches up to seven degrees above the horizon, allowing many polar features hidden from the sun to be imaged by radar.

The floors of five large craters in the south polar region are hidden from the sun, the researchers say. These five crater floors constitute the largest potential deposits of water ice at the south pole and would be expected to display an excess of hydrogen if they contained ice.

Radar record

The scientists making the maps used the Goldstone 70 metre (230 feet) antenna to transmit the radar signals. Two 34 m (110 ft) antennas at the Goldstone site received the echoes.

Scientists derived a three-dimensional digital elevation model of the lunar poles by comparing the images from the two antennas, which are 20 kilometres (12 miles) apart.

Their three dimensional maps have measurements every 150 m (500 ft) over the imaged area and a height accuracy of 50 m (165 ft).